In the microfabrication using a resist composition, with an increase in the integration degree of an integrated circuit, formation of an ultrafine pattern is required. To meet this requirement, the exposure wavelength tends to become shorter and, for example, development of a lithography technique using an electron beam, an X-ray or EUV light in place of excimer laser light is proceeding.
In order to form such an ultrafine pattern, thin film formation of the resist is necessary, but when the resist is formed as a thin film, its dry etching resistance is sometimes impaired.
In the electron beam (EB) lithography, it is known that the effect of electron scattering in the resist film, that is, forward scattering, is reduced by increasing the accelerating voltage of EB. For this reason, recently, there is a tendency to increase the accelerating voltage of EB. However, when the accelerating voltage of EB is increased, the resist film is sometimes reduced in the electron energy trapping ratio and in turn, in the sensitivity.
As one of methods to solve such a problem, use of a resin having a polycyclic aromatic structure, such as naphthalene, is being studied (see, for example, JP-A-2008-95009 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), International Publication No. 2007-046453, JP-A-2008-50568, JP-A-2008-268871, JP-A-2009-86354). By using a resin having a polycyclic aromatic structure, for example, dry etching resistance and sensitivity can be enhanced.
When the accelerating voltage of EB is increased, the effect of forward scattering becomes small, but the effect of scattering of electrons reflected on the resist substrate, that is, back scattering, increases. This effect of back scattering is great particularly when forming an isolated pattern having a large exposure area. Therefore, the increase of accelerating voltage of EB may cause, for example, reduction in the resolution of an isolate pattern.
Particularly, in the case of patterning a photomask blank used for semiconductor exposure, a light-shielding film containing a heavy atom such as chromium, molybdenum and tantalum is present as a resist underlayer and compared with the case of coating a resist on a silicon wafer, the effect of back scattering attributable to reflection from the resist underlayer is more prominent. Therefore, formation of an isolated pattern on a photomask blank is susceptible to the effect of back scattering and the resolution is highly likely to decrease.
As one of the methods to enhance the resolution of an isolated pattern, use of a resin containing a group for adjusting the solubility of the resin is being studied (see, for example, Japanese Patent No. 3,843,115, JP-A-2005-99558), but it has not been achieved to sufficiently satisfy the resolution and rectangularity of an isolated pattern.
In late years, other than the excimer laser light, development of lithography using an electron beam, an X-ray or EUV light is proceeding. Furthermore, microfabrication by a resist composition is applied not only directly to the production of an integrated circuit but recently also to the fabrication of a so-called imprint mold structure (see, for example, JP-A-2004-158287, JP-A-2008-162101, Yoshihiko Hirai (compiler), Nanoimprint no Kiso to Gijutsu Kaihatsu/Oyo Tenkai-Nanoimprint no Kiban Gijutsu to Saishin no Gijutsu Tenkai (Basic and Technology Expansion/Application Development of Nanoimprint-Substrate Technology of Nanoimprint and Latest Technology Expansion), Frontier Shuppan (issued June, 2006)). Therefore, particularly in the case of forming an isolated pattern by using an X-ray, a soft X-ray or an electron beam for the exposure light source, it is also an important task to simultaneously satisfy high sensitivity, high resolution and good dry etching resistance, and this needs to be solved.